The superrotation of solar supergranules

Meunier, N.; Roudier, Th.

doi:10.1051/0004-6361:20066790

Cited by 16 publications

(12 citation statements)

References 18 publications

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“…The measured anisotropy of the magnetic field is but one result in a series of puzzling observations associated with supergranulation. Other observations include, for example, the finding that the supergranulation pattern rotates faster around the Sun than magnetic features (e.g., Snodgrass & Ulrich 1990;Meunier & Roudier 2007) and the wavelike properties of supergranulation (Gizon et al 2003;Schou 2003). How these phenomena are related to the discovery in this paper is unclear.…”

Section: Resultsmentioning

confidence: 84%

Anisotropy of the solar network magnetic field around the average supergranule

Langfellner

Gizon

Birch

2015

A&A

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Supergranules in the quiet Sun are outlined by a web-like structure of enhanced magnetic field strength, the so-called magnetic network. We aim to map the magnetic network field around the average supergranule near disk center. We use observations of the line-of-sight component of the magnetic field from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The average supergranule is constructed by coaligning and averaging over 3000 individual supergranules. We determine the positions of the supergranules with an image segmentation algorithm that we apply to maps of the horizontal flow divergence measured using time-distance helioseismology. In the center of the average supergranule, the magnetic (intranetwork) field is weaker by about 2.2 Gauss than the background value (3.5 Gauss), whereas it is enhanced in the surrounding ring of horizontal inflows (by about 0.6 Gauss on average). We find that this network field is significantly stronger west (prograde) of the average supergranule than in the east (by about 0.3 Gauss). With time-distance helioseismology, we find a similar anisotropy. The observed anisotropy of the magnetic field adds to the mysterious dynamical properties of solar supergranulation.

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Section: Resultsmentioning

confidence: 84%

Anisotropy of the solar network magnetic field around the average supergranule

Langfellner

Gizon

Birch

2015

A&A

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“…Even more recently, Hathaway et al (2006) argued that the supergranulation pattern superrotation inferred from Doppler shifts was due to projection effects on the line-of-sight signal. Using correlation tracking of divergence maps derived from intensity maps (Meunier et al, 2007c) and comparing it with direct Doppler tracking, Meunier and Roudier (2007) confirmed the existence of projection effects with the latter method, but found that the supergranulation pattern inferred from divergence maps was still superrotating, albeit at smaller angular velocities than those inferred by Duvall Jr (1980) and Snodgrass and Ulrich (1990). For a detailed discussion on the identification of supergranulation rotational properties with helioseismology, we refer the reader to the review article by Gizon and Birch (2005) on local helioseismology.…”

Section: Rotational Properties Of Supergranulesmentioning

confidence: 72%

The Sun’s Supergranulation

Rincon

Rieutord

2010

Living Rev. Solar Phys.

165

132

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The Sun's supergranulation refers to a physical pattern covering the surface of the quiet Sun with a typical horizontal scale of approximately 30,000 km and a lifetime of around 1.8 d. Its most noticeable observable signature is as a fluctuating velocity field of 360 m s -1 rms whose components are mostly horizontal. Supergranulation was discovered more than fifty years ago, however explaining why and how it originates still represents one of the main challenges of modern solar physics.A lot of work has been devoted to the subject over the years, but observational constraints, conceptual difficulties and numerical limitations have all concurred to prevent a detailed understanding of the supergranulation phenomenon so far. With the advent of 21st century supercomputing resources and the availability of unprecedented high-resolution observations of the Sun, a stage at which key progress can be made has now been reached. A unifying strategy between observations and modelling is more than ever required for this to be possible.The primary aim of this review is therefore to provide readers with a detailed interdisciplinary description of past and current research on the problem, from the most elaborate observational strategies to recent theoretical and numerical modelling efforts that have all taken up the challenge of uncovering the origins of supergranulation. Throughout the text, we attempt to pick up the most robust findings so far, but we also outline the difficulties, limitations and open questions that the community has been confronted with over the years.In the light of the current understanding of the multiscale dynamics of the quiet photosphere, we finally suggest a tentative picture of supergranulation as a dynamical feature of turbulent magnetohydrodynamic convection in an extended spatial domain, with the aim of stimulating future research and discussions.

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“…We have tracked supergranules as described in Meunier & Roudier (2007). This tracking is a combination of local correlation and feature tracking, as the local correlation is computed only at places where the divergence is maximum: we therefore track the positions of maximum divergence.…”

Section: Evolution Of the Cellsmentioning

confidence: 99%

Are supergranule sizes anti-correlated with magnetic activity?

Meunier

Roudier

Tkaczuk

2007

A&A

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Context. The variation of supergranule cell sizes with the magnetic environment is still controversial. Aims. We study this relation in detail to understand the discrepancies observed between previous results. Methods. We determine the cell size using divergence of horizontal flows derived from local correlation tracking of intensity maps (MDI/SOHO). We study the variation of the cell size as a function of the magnetic field inside the cell. We also consider which component of the magnetic field most influences the cell size. Results. Our main conclusion is that there are no large cells when the magnetic field (in absolute value) averaged over the cell is large. This is mostly due to the magnetic field inside the cell (intranetwork fields), while strong network magnetic fields (at the cell boundary) are associated with larger cells. Further studies of the evolution of the cells and of the flux imbalance suggest that a high level of weak fields may prevent the formation of large cells. This is compatible with the expectation that strong magnetic fields should prevent large-scale flows. Conclusions. The relation between the local activity level determined by the average magnetic field inside the cells and the supergranule size is not linear. Furthermore, it strongly depends on the definition of the activity level (magnetic field inside the cell or magnetic network) and on the magnetic sensitivity of the data. This last point probably explains at least partially the conflicting results obtained up to now.

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The superrotation of solar supergranules

Cited by 16 publications

References 18 publications

Anisotropy of the solar network magnetic field around the average supergranule

Anisotropy of the solar network magnetic field around the average supergranule

The Sun’s Supergranulation

Are supergranule sizes anti-correlated with magnetic activity?

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